Fusion power supply harmonic suppression method based on energy storage type STATCOM

Abstract

The invention relates to a fusion power supply harmonic suppression method based on an energy storage type STATCOM. The method comprises the following steps: determining a harmonic amplification factor K value; selecting and designing a proper discrete digital band-pass filter, and detecting a target frequency band and filtering characteristic sub-harmonics; the detected harmonic voltage amplitude and phase are used as references, an energy storage type STATCOM is adopted to output same-frequency and same-phase harmonic current, virtual damping is formed at a grid-connected point, a K value is used as a damping coefficient, the output harmonic current reference amplitude is determined, virtual harmonic damping is controlled, and harmonic resonance is suppressed. The virtual harmonic damping method for the energy storage type STATCOM is provided for accurate tracking of low-frequency harmonic and inter-harmonic voltage amplitudes and phases, the chain type STATCOM has the advantages of being high in high-voltage direct-hanging efficiency, high in redundancy fault-tolerant capability and the like and is suitable for high-voltage large-capacity scenes, virtual damping control achieves harmonic suppression by adjusting virtual damping at the harmonic frequency of a control structure, and the energy storage type STATCOM can be applied to high-voltage large-capacity scenes. And the system stability can be effectively improved while harmonic waves are treated.

Description

Technical Field

[0001] The present invention relates to the field of electric power technology, and in particular to a method for controlling harmonics of a fusion power source based on an energy storage type STATCOM. Background Art

[0002] Fusion power sources will produce large-amplitude, rapidly changing second harmonics and abundant interharmonic components near the fundamental frequency. Their extraction and control methods are the key to real-time, effective and targeted compensation.

[0003] The traditional instantaneous reactive power and adaptive detection methods obtain the harmonic compensation signal by subtracting the extracted fundamental frequency signal from the grid signal, which is difficult to effectively solve the problem of extracting fixed multi-frequency harmonic signals. The wavelet analysis method has good time-frequency characteristics and can effectively detect sudden harmonics and obtain more accurate harmonic parameters, but its application is limited by its shortcomings such as long sampling time, difficulty in matching wavelet basis, and inability to effectively separate similar frequencies. The FIR-based digital bandpass detection algorithm effectively extracts low-frequency interharmonics in the fusion power system, but it has the problems of high transfer function order and large amount of calculation, and has not yet been actually applied in the power system.

[0004] Traditional harmonic control ideas are mainly passive filtering solutions, which have a good suppression effect on characteristic subharmonics, but cannot effectively control low-frequency harmonics and interharmonics that change rapidly and frequently and easily affect the fundamental frequency signal. The active filtering solution proposed in recent years detects the amplitude and phase of the harmonic current and outputs a harmonic current with the same amplitude and opposite phase as the detection signal. Although it has achieved certain results, it has high requirements on the capacity and control of the compensation device, and will limit the high-power and long-pulse operation capabilities of the fusion power supply. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a method for harmonic control of a fusion power source based on an energy storage type STATCOM, so as to overcome the deficiencies in the above-mentioned prior art.

[0006] The technical solution of the present invention to solve the above technical problems is as follows: A method for harmonic control of fusion power source based on energy storage type STATCOM, comprising the following steps:

[0007] Step S01: Determine the harmonic amplification factor K value by combining the harmonic voltage and current data of the fusion power system during operation and the national standard limit values ​​of harmonics of each frequency at different voltage levels;

[0008] Step S02: Select and design a suitable discrete digital bandpass filter, extract the second harmonic and interharmonics close to the high-amplitude fundamental frequency signal, detect the target frequency band and filter out the characteristic subharmonics;

[0009] Step S03: Based on the detection method of discrete double-synchronous decoupling, accurately track the voltage amplitude and phase of low-frequency harmonics and inter-harmonics;

[0010] Step S04: Use the detected harmonic voltage amplitude and phase as a reference, and adopt an energy storage type STATCOM to output harmonic currents with the same frequency and phase. Form a virtual damping at the connection point, use the K value as the damping coefficient, determine the reference amplitude of the output harmonic current, and perform virtual harmonic damping control to suppress harmonic resonance.

[0011] The beneficial effects of the present invention are as follows: Based on the detection methods of discrete digital band-pass filters and discrete double-synchronous decoupling, the present invention realizes accurate tracking of the voltage amplitude and phase of low-frequency harmonics and inter-harmonics. Combining the resonance amplification laws of low-frequency harmonics and inter-harmonics, a virtual harmonic damping method for an energy storage type STATCOM is proposed. The cascaded STATCOM has the characteristics of high efficiency in directly connecting to high voltage and strong redundancy and fault tolerance capabilities, and is suitable for high-voltage and large-capacity scenarios. The virtual damping control realizes harmonic suppression by adjusting the virtual damping at the harmonic frequency of the control structure, and can effectively improve the system stability while treating harmonics.

[0012] On the basis of the above technical solutions, the present invention can also be improved as follows.

[0013] Further, step S02 specifically includes the following steps:

[0014] Step S21: Design relevant parameters and perform parameter tuning;

[0015] Step S22: After parameter tuning, select various types of band-pass filters and call the corresponding functions to obtain their respective transfer functions through iteration;

[0016] Step S23: Determine whether the design requirements are met. If the design requirements are not met, return to step S21 and step S22 to execute again; if the design requirements are met, select it as the optimal band-pass filter.

[0017] Further, step S21 specifically includes the following steps:

[0018] Step S211: Set the initial values of the passband attenuation R p , stopband attenuation R s , sampling frequency F s , left boundary frequency of the passband and right boundary frequency of the passband ;

[0019] Step S212: Substitute the initial values into the formula and adjust the adjustable frequency Δσ i ;

[0020] Step S213: Based on the adjustable frequency Δσ i, calculate the passband boundary frequency ω p and the stopband cut-off frequency ω s .

[0021] Furthermore, the setting formula for the initial value in step S211 is:

[0022]

[0023] In the formula, R p is the passband attenuation, R s is the stopband attenuation, ω p is the passband boundary frequency, ω s is the stopband cut-off frequency, F s is the sampling frequency, is the left boundary frequency of the passband, is the right boundary frequency of the passband, Δσ i is the adjustable frequency, and N0 is the target order of the filter design.

[0024] Furthermore, the calculation formulas for the passband boundary frequency ω p and the stopband cut-off frequency ω s in step S213 are as follows:

[0025]

[0026] Furthermore, step S03 specifically includes the following steps:

[0027] Step S31: Define trigonometric functions:

[0028]

[0029] where pω and nω represent that any voltage vector V rotates at angular velocities of pω and nω respectively;

[0030] Step S32: The decoupled network is expressed as:

[0031]

[0032] Step S33: Further obtain the detection phase expression as:

[0033]

[0034] Furthermore, step S04 specifically includes the following steps:

[0035] Step S41: Perform DC-side voltage stabilization control based on the voltage outer-loop control method;

[0036] Compare the per-unit value of the DC-side voltage reference with the per-unit value of the average DC-side voltage of the sub-module And perform PI regulation on the difference between the two to obtain the reference value of the d-axis current. The control transfer function of the voltage outer loop is:

[0037]

[0038] In the formula, is the per-unit value of the DC-side voltage reference, is the per-unit value of the average DC-side voltage of the sub-module. Among them, U dc_ref is the DC-side voltage reference value, U dc_B is the DC-side voltage reference value, U dc_av is the average DC-side voltage of the sub-module, K pdc is the DC-side proportional gain, K idc is the DC-side integral gain;

[0039] Step S42: Voltage feed-forward current feedback control;

[0040] The control transfer function of the current inner loop is:

[0041]

[0042] Among them, i hd (s) is the voltage feed-forward current, G PI (s) is the transfer function of the current inner loop PI controller, i hd_ref =U hd / K is the current reference value, K is the damping coefficient, k fv is the voltage feed-forward coefficient, K PWM is the AC-DC modulation ratio;

[0043] Reduce the coefficient of the disturbance voltage u hd (s) to 0 through voltage feed-forward control, that is:

[0044] 1-k fv K PWM =0;

[0045] Since the AC-DC modulation ratio k fv is the ratio of the AC voltage to the DC voltage, therefore:

[0046]

[0047] Among them, U d_peak is the peak value of the AC-side voltage;

[0048] At this time, the voltage feed-forward current is:

[0049] Description of the Drawings

[0050] Figure 1Flow chart of harmonic suppression based on energy storage type STATCOM of the present invention;

[0051] Figure 2 Flow chart of Cheby Ⅱ - DDSD harmonic detection of the present invention;

[0052] Figure 3 Amplitude - frequency and phase - characteristic curve of Chebyshev - Ⅱ type digital band - pass filter of the present invention;

[0053] Figure 4 DDSD phase - detection method diagram of the present invention;

[0054] Figure 5 Harmonic damping control block diagram based on energy storage type STATCOM of the present invention;

[0055] Figure 6 Voltage outer - loop control block diagram of the present invention;

[0056] Figure 7 Current inner - loop control block diagram with voltage feed - forward control introduced of the present invention;

[0057] Figure 8 Output current curve with / without voltage feed - forward of the present invention;

[0058] Figure 9 Cheby Ⅱ - DDSD harmonic detection experimental curve of the present invention;

[0059] Figure 10 Output curve of second - harmonic prototype of the present invention;

[0060] Figure 11 Physical diagram of energy storage type STATCOM prototype of the present invention Specific implementation mode

[0061] The principles and features of the present invention will be described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0062] As Figures 1 to 11 shown, in Embodiment 1, a harmonic suppression method for a fusion power supply based on an energy storage type STATCOM includes the following steps:

[0063] Step S01: Combine the harmonic voltage and current data during the operation of the fusion power supply system and the national standard limits of each frequency harmonic under different voltage levels to determine the harmonic amplification factor K value;

[0064] Step S02: Select and design a suitable discrete digital band - pass filter to extract the second - harmonic and inter - harmonic waves in the high - amplitude fundamental - frequency signal, and detect the target frequency band and filter out the characteristic sub - harmonic waves;

[0065] Step S03: Based on the detection method of discrete double synchronous decoupling, accurately track the voltage amplitude and phase of low-frequency harmonics and inter-harmonics.

[0066] Step S04: Taking the detected harmonic voltage amplitude and phase as references, use the energy storage type STATCOM to output harmonic currents with the same frequency and phase, form virtual damping at the grid connection point, use the K value as the damping coefficient, determine the reference amplitude of the output harmonic current, and perform virtual harmonic damping control to suppress harmonic resonance.

[0067] The present invention is based on the detection method of discrete digital band-pass filter and discrete double synchronous decoupling (ChebyⅡ-DDSD) to achieve accurate tracking of the voltage amplitude and phase of low-frequency harmonics and inter-harmonics. Combining the resonance amplification laws of low-frequency harmonics and inter-harmonics, a virtual harmonic damping method for the energy storage type STATCOM is proposed. The chain type STATCOM has the characteristics of high efficiency of directly connecting to high voltage, strong redundancy and fault tolerance ability, etc., and is suitable for high-voltage and large-capacity scenarios. The virtual damping control realizes harmonic suppression by adjusting the virtual damping at the harmonic frequency of the control structure, and can effectively improve the system stability while treating harmonics.

[0068] Embodiment 2 is a further improvement based on Embodiment 1, which is specifically as follows:

[0069] Step S02 specifically includes the following steps:

[0070] Step S21: Design relevant parameters and perform parameter tuning.

[0071] Step S22: After parameter tuning, select various types of band-pass filters and call the corresponding functions to obtain their respective transfer functions through iteration.

[0072] Step S23: Determine whether the design requirements are met. If the design requirements are not met, return to re-execute Step S21 and Step S22; if the design requirements are met, select it as the optimal band-pass filter.

[0073] Embodiment 3 is a further improvement based on Embodiment 2, which is specifically as follows:

[0074] Step S21 specifically includes the following steps:

[0075] Step S211: Set the initial values of the passband attenuation R p , stopband attenuation R s , sampling frequency F s , left boundary frequency of the passband and right boundary frequency of the passband .

[0076] Step S212: Substitute the initial value into the formula and adjust the adjustable frequency Δσ i ;

[0077] Step S213: Based on the adjustable frequency Δσ i , calculate the passband boundary frequency ω p and the stopband cut-off frequency ω s .

[0078] Example 4. This example is a further improvement based on Example 3, and the details are as follows:

[0079] To extract the second harmonics and interharmonics in the high-amplitude fundamental frequency signal, it is necessary to design reasonable pass, stop, and transition band boundaries, and establish an improved amplitude-phase model in combination with different detection requirements. In the design process of the digital band-pass filter, the four most critical elements include the passband attenuation R p , the stopband attenuation R s , the passband boundary frequency ω p and the stopband cut-off frequency ω s ; As known from the filter characteristics, the passband range determines the detection effect of the target frequency band and the filtering effect of the characteristic sub-harmonics. Therefore, the formula for setting the initial value in Step S211 is:

[0080]

[0081] In the formula, R p is the passband attenuation, R s is the stopband attenuation, ω p is the passband boundary frequency, ω s is the stopband cut-off frequency, F s is the sampling frequency, is the left boundary frequency of the passband, is the right boundary frequency of the passband, Δσ i is the adjustable frequency, and N0 is the target order of the filter design.

[0082] Example 5. This example is a further improvement based on Example 4, and the details are as follows:

[0083] The calculation formulas for the passband boundary frequency ω p and the stopband cut-off frequency ω s in Step S213 are as follows:

[0084]

[0085] For example: According to the cheby2 function, the transfer function of the Chebyshev-II type digital band-pass filter can be obtained through the iterative process shown in Figure 2 as follows:

[0086]

[0087] In the formula, the coefficients of the denominator and numerator polynomials are shown in Table 1. Under these parameters, the amplitude-frequency and phase characteristics of the Chebyshev-II type digital band-pass filter are as Figure 3 shown.

[0088] Table 1 Transfer function coefficients

[0089]

[0090] Example 6 is a further improvement based on Example 1, and the specific content is as follows:

[0091] Considering that the low-frequency harmonic phase of the fusion power supply system changes rapidly and is vulnerable to the impact of impulse loads, and the three-phase imbalance phenomenon occurs in the power grid due to negative sequence current, a phase detection method based on discrete double synchronous decoupling (DDSD) is adopted, which can effectively overcome the problems of inaccurate phase detection and slow response caused by the power grid frequency change. The principle of interharmonic phase detection is as Figure 4 shown, and this method can detect the harmonic phase accurately under extreme conditions such as load impact and three-phase imbalance of the power grid.

[0092] Step S03 specifically includes the following steps:

[0093] Step S31: Define trigonometric functions:

[0094]

[0095] where pω and nω represent that any voltage vector V rotates at angular velocities of pω and nω respectively;

[0096] Step S32: The decoupling network is expressed as:

[0097]

[0098] Step S33: Further obtain the detection phase expression as:

[0099]

[0100] Example 7 is a further improvement based on Example 1, and the specific content is as follows:

[0101] Based on the detection of harmonic voltage amplitude and phase, research is carried out on the control method of the energy storage type STATCOM. The control method of voltage feed-forward current feedback and voltage outer loop is adopted. The system topology and control strategy are as Figure 5 shown.

[0102] Step S04 specifically includes the following steps:

[0103] Step S41: Perform DC side voltage stabilization control based on the voltage outer loop control method;

[0104] Considering that the inter-harmonic signal has a great influence on the DC voltage stabilization of the cascaded STATCOM, energy storage elements are added to the original topology, which helps to stabilize the DC side voltage. On this basis, a voltage outer loop control method is designed. By comparing the real-time voltage average value with the reference voltage and performing PI adjustment on the difference, the reference value of the d-axis current is obtained, and then the control of the output harmonic current is realized. The specific control block diagram is as Figure 6 shown.

[0105] Compare the per-unit value of the DC side voltage reference with the per-unit value of the average DC side voltage of the sub-module and perform PI adjustment on the difference between the two to obtain the reference value of the d-axis current. The control transfer function of the voltage outer loop is:

[0106]

[0107] In the formula, is the per-unit value of the DC side voltage reference, is the per-unit value of the average DC side voltage of the sub-module, where U dc_ref is the DC side voltage reference value, U dc_B is the DC side voltage reference value, U dc_av is the average DC side voltage of the sub-module, K pdc is the DC side proportional gain, K idc is the DC side integral gain;

[0108] Step S42: Voltage feedforward current feedback control;

[0109] Power shocks will occur during the magnet power conversion process, the rapid protection of the auxiliary heating power supply, and the start-up moment of the energy storage device, etc., triggering harmonics with rich side lobes and rapid changes. To effectively reduce its impact on the fundamental frequency signal and the grid voltage, voltage feedforward control is introduced, and a voltage feedforward current feedback control method with small computational amount, low complexity, and good current response performance is adopted.

[0110] Figure 7 The voltage feedforward current feedback control block diagram for compensating harmonics is given, and the control transfer function of the current inner loop is:

[0111]

[0112] Among them, i hd (s) is the voltage feedforward current, G PI (s) is the transfer function of the current inner loop PI controller, i hd_ref =U hd / K is the current reference value, K is the damping coefficient, and k fv is the voltage feedforward coefficient, and K PWM is the AC-DC modulation ratio;

[0113] If the voltage feedforward link is not added, that is, the voltage feedforward coefficient k fv = 0, the transfer function of the inner current loop control is shown as follows:

[0114]

[0115] It can be seen from the above formula that the output current of the cascaded energy storage device will be affected by the grid voltage disturbance. When a power shock occurs, the strong fluctuation of the grid voltage will cause obvious distortion of the output current of the cascaded energy storage device. Therefore, the voltage feedforward link is essential.

[0116] To avoid the distortion of the output current caused by the power shock, the coefficient of the disturbance voltage u hd (s) is reduced to 0 through voltage feedforward control, that is:

[0117] 1 - k fv K PWM = 0;

[0118] Since the AC-DC modulation ratio k fv is the ratio of the AC voltage to the DC voltage, therefore:

[0119]

[0120] where U d_peak is the peak value of the AC side voltage;

[0121] At this time, the voltage feedforward current is:

[0122]

[0123] 1. Simulation verification: To verify the effectiveness of the voltage feedforward current feedback control method, the output currents with and without voltage feedforward control were simulated and analyzed. As Figure 8 shown, in the case without the voltage feedforward link, the transient harmonic current impact at the moment of starting the energy storage type STATCOM is large, which is very likely to cause the device to exceed the limit protection and interrupt the operation. After adding the voltage feedforward link, the current impact can be significantly reduced, ensuring the safe and reliable operation of the energy storage type STATCOM.

[0124] 2. Prototype experiment verification: Based on the above theoretical analysis of the interharmonic detection between Cheby Ⅱ-DDSD and the virtual damping method based on E-STATCOM, in this part, an energy storage type cascaded STATCOM prototype was first developed, mainly including the cascaded STATCOM part, energy storage unit, three-phase power grid, and measuring equipment such as Figure 11As shown in the figure. In addition, the H-bridge DC side of each module and the corresponding energy storage unit are optimized and matched. The energy storage module is composed of 10 battery cells connected in series (the capacity of each battery cell is 2Ah). The design of the key parameters of the prototype is shown in Table 2:

[0125] Table 2 Parameters of the experimental prototype

[0126]

[0127]

[0128] 2.1 Experimental verification of the ChebyⅡ-DDSD (harmonic phase detection based on double synchronous decoupling) detection method:

[0129] Based on Figure 11 the energy storage type STATCOM prototype shown in the figure, in order to verify the performance of the ChebyⅡ-DDSD detection algorithm, for the complex and variable low-frequency inter-harmonic signals, taking the second harmonic as an example, the experiment is as follows:

[0130] (1) Second harmonic detection following performance experiment. Input the reference second harmonic voltage into the ChebyⅡ-DDSD detection algorithm, and the voltage curves of the input and output are as Figure 9 (a) shown. It can be seen that the detection accuracy of the second harmonic is as high as 92.8%, and the steady-state error is only 0.8%;

[0131] (2) Under the condition of multi-harmonic source interference, verify the second harmonic detection performance of the ChebyⅡ-DDSD algorithm. Input the reference multi-harmonic source voltage signal (the superimposed signal of the 2nd, 3rd, 5th, and 7th harmonics) into the proposed detection algorithm, and the voltage curves of the input and output are as Figure 9 (b) shown. The detection algorithm can accurately extract the second harmonic, and the accuracy is as high as 91.2%;

[0132] (3) Second harmonic phase detection experiment. The phase after passing through the ChebyⅡ-DDSD detection algorithm is as Figure 9 (c) shown, and accurate phase locking can be achieved within one cycle.

[0133] 2.2 Verification of the control method of the energy storage type STATCOM:

[0134] Taking the typical experiment 59851 shots of the EAST device as an example, during the operation, through the power quality analyzer, it is measured that the amplitude of the second harmonic is nearly 10 times larger than the national standard limit. Therefore, taking 0.1*Uh as the reference value of the second harmonic output current, control the energy storage type STATCOM, and the output harmonic voltage and current curves are as Figure 10As shown, the second harmonic output current is 10% of the voltage, and the phases of the voltage and current are the same. The prototype can accurately output harmonic currents with specified amplitudes, frequencies, and phases, form virtual harmonic damping at the point of common coupling, effectively suppress harmonic resonance, and verify the correctness of the control method.

[0135] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for harmonic control of fusion power source based on energy storage STATCOM, characterized in that: The steps include: Step S01: Determine the harmonic amplification factor K value by combining the harmonic voltage and current data of the fusion power system during operation and the national standard limit values ​​of harmonics of each frequency at different voltage levels; Step S02: Select and design a suitable discrete digital bandpass filter, extract the second harmonic and interharmonics close to the high-amplitude fundamental frequency signal, detect the target frequency band and filter out the characteristic subharmonics; Step S03: Based on the detection method of discrete dual synchronous decoupling, the voltage amplitude and phase of low-frequency harmonics and interharmonics are accurately tracked; Step S04: Using the detected harmonic voltage amplitude and phase as a reference, an energy storage type STATCOM is used to output harmonic currents of the same frequency and phase, forming virtual damping at the grid connection point, and using the K value as the damping coefficient to determine the reference amplitude of the output harmonic current, control the virtual harmonic damping, and suppress harmonic resonance.

2. A method for controlling harmonics of a fusion power source based on an energy storage type STATCOM according to claim 1, characterized in that: The step S02 specifically includes the following steps: Step S21: design relevant parameters and perform parameter setting; Step S22: After the parameters are set, multiple types of bandpass filters are selected and the corresponding functions are called to obtain respective transfer functions through iteration; Step S23: Determine whether the design requirements are met. If not, return to re-execute the steps S21 and S22; if the design requirements are met, select it as the optimal bandpass filter.

3. A method for controlling harmonics of a fusion power source based on an energy storage type STATCOM according to claim 2, characterized in that: The step S21 specifically includes the following steps: Step S211: Setting the passband attenuation R p , Stopband attenuation R s , sampling frequency F s , passband left edge frequency and the right edge frequency of the passband The initial value of Step S212: Substitute the initial value into the formula and adjust the adjustable frequency Δσ i ; Step S213: Based on the adjustable frequency Δσ i , calculate the passband edge frequency ω p and stopband cutoff frequency ω s .

4. A method for controlling harmonics of a fusion power source based on an energy storage type STATCOM according to claim 3, characterized in that: The setting formula of the initial value in step S211 is: In the formula, R p is the passband attenuation, R s is the stopband attenuation, ω p is the passband edge frequency, ω s is the stopband cutoff frequency, F s is the sampling frequency, is the passband left edge frequency, is the right edge frequency of the passband, Δσ i is the adjustable frequency, and N0 is the target order of the filter design.

5. A method for controlling harmonics of a fusion power source based on an energy storage type STATCOM according to claim 4, characterized in that: The passband boundary frequency ω in step S213 p and the stopband cutoff frequency ω s The calculation formula is as follows:

6. The method for harmonic control of fusion power source based on energy storage STATCOM according to claim 1, characterized in that: The step S03 specifically includes the following steps: Step S31: define trigonometric functions: Wherein, pω and nω represent that any voltage vector V rotates at the angular velocity of pω and nω respectively; Step S32: The decoupling network is expressed as: Step S33: further obtain the detection phase expression as:

7. The method for harmonic control of fusion power source based on energy storage STATCOM according to claim 1, characterized in that: The step S04 specifically includes the following steps: Step S41: performing DC side voltage stabilization control based on a voltage outer loop control method; Compare DC voltage reference per unit value The per unit value of the average DC voltage of the submodule The difference between the two is adjusted by PI to obtain the d-axis current reference value. The control transfer function of the voltage outer loop is: In the formula, is the DC side voltage reference per unit value, is the per unit value of the average DC side voltage of each submodule, where U dc_ref is the DC side voltage reference value, U dc_B is the DC side voltage reference value, U dc_av is the average DC side voltage of the submodule, K pdc is the DC side proportional gain, K idc is the DC side integral gain; Step S42: voltage feedforward and current feedback control; The current inner loop control transfer function is: Among them, i hd (s) is the voltage feedforward current, G PI (s) is the transfer function of the current inner loop PI controller, i hd_ref =U hd / K is the current reference value, K is the damping coefficient, k fv is the voltage feed-forward coefficient, K PWM is the AC / DC modulation ratio; The disturbance voltage u is controlled by voltage feedforward. hd The coefficient of (s) drops to 0, that is: 1-k fv K PWM =0; Since the AC / DC modulation ratio k fv is the ratio of AC voltage to DC voltage, so: Among them, U d_peak is the peak voltage on the AC side; At this time, the voltage feed-forward current is:

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